Conventional methods for drycleaning use a chlorinated hydrocarbon solvent, most commonly perchloroethylene (PERC) in combination with small amounts of water and detergents. While PERC is fabric-safe, non-flammable, and easily recycled, it has come under attack in recent years as an environmental and health hazard. In particular, PERC is listed as a Hazardous Air Pollutant (HAP), it is non-biodegradable, and it is a probable human carcinogen.
In recent years, the industry has responded with less-toxic alternatives to PERC, including hydrocarbons (e.g., EcoSolv™ drycleaning fluid from CPChem) and glycol ethers. We recently found (see copending application Ser. No. 10/653,725) that compositions that contain at least 80 wt. % dipropylene glycol n-propyl ether (DPnP) and up to about 15 wt. % water are exceptionally useful for drycleaning. Moreover, we found that there is no need to use DPnP in combination with polysulfonic acids or cyclic siloxanes as is taught elsewhere (see, e.g., U.S. Pat. Nos. 6,086,634 and 6,042,617).
Other glycol ethers have been recommended for use in drycleaning, most notably propylene glycol tert-butyl ether (PTB), propylene glycol n-butyl ether (PNB), dipropylene glycol tert-butyl ether (DPTB) and dipropylene glycol n-butyl ether (DPNB). See, for example, U.S. Pat. Nos. 5,888,250, 6,156,074, 6,273,919, and 6,350,287, all assigned to Rynex Holdings, Ltd. In particular, the '919 and '287 patents teach DPTB as an alternative with significant advantages over PERC. DPTB has a high flash point and good detergency. The compositions taught for use are DPTB/water (>9:1 by weight) mixtures. The use of glycol ethers, including DPTB, represents a significant step toward replacing PERC in drycleaning.
The ability of the solvent to solubilize enough water is another concern. Ideally, the solvent will have the ability to solubilize at least about 4 wt. % of water. There is a balance to strike, however, because a solvent holding too much water can promote more than a desirable amount of shrinkage.
With both alkyl ether and hydroxy end groups, glycol ethers such as DPTB and DPNB are amphiphilic. Consequently, they can be challenging to separate quickly and completely from a relatively small proportion of water. For example, water-saturated DPTB contains about 10 wt. % of water. An ideal drycleaning solvent will hold only about 5 wt. % of water when saturated. Moreover, an ideal solvent will separate rapidly and completely from the water extracted from fabrics during drycleaning.
Another drawback of some glycol ethers, particularly ones based on di- or tripropylene glycols, is their slow evaporation rate. For example, DPTB evaporates only 1.2% as fast as n-butyl acetate. Faster evaporation means higher productivity and profitability for a drycleaning business. Moreover, higher boiling glycol ethers are more costly to reclaim by distillation.
Some glycol ethers proposed for drycleaning have an undesirably low flash point, i.e., one that is near or below room temperature on a hot day. For example, PTB has a flash point (Tag, closed cup) of only 45° C. A minimum flash point of about 60° C. or higher would be preferable.
Dipropylene glycol dimethyl ether (DMM) is commercially available. It has been used in detergents (see, e.g., U.S. Pat. No. 6,696,399), polyurethane dispersions (U.S. Pat. No. 6,541,536), and polymer stripping compositions (U.S. Pat. No. 6,455,479). DMM has not been specifically mentioned as being useful for drycleaning.
Good progress has been made to date, but the industry continues to need replacements for PERC. In particular, the industry would benefit a drycleaning composition having a relatively high evaporation rate combined with an acceptable flash point. An improved drycleaning method would be effective for both oily and more water-soluble soils. An ideal cleaner would use readily available, inexpensive components, would rival or outperform PERC and its commercial alternatives, and would have a favorable water solubility profile. Finally, the drycleaning method must not harm the fabric. In particular, the method must not cause undue shrinkage (i.e., more than about 2%).